I have chanced upon an instance of exceedingly erratic performance profile of a very simple map/reduce operation on primitive arrays. Here is my jmh benchmark code:
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@BenchmarkMode(Mode.AverageTime)
@OperationsPerInvocation(Measure.ARRAY_SIZE)
@Warmup(iterations = 300, time = 200, timeUnit=MILLISECONDS)
@Measurement(iterations = 1, time = 1000, timeUnit=MILLISECONDS)
@State(Scope.Thread)
@Threads(1)
@Fork(1)
public class Measure
{
static final int ARRAY_SIZE = 1<<20;
final int[] ds = new int[ARRAY_SIZE];
private IntUnaryOperator mapper;
@Setup public void setup() {
setAll(ds, i->(int)(Math.random()*(1<<7)));
final int multiplier = (int)(Math.random()*10);
mapper = d -> multiplier*d;
}
@Benchmark public double multiply() {
return Arrays.stream(ds).map(mapper).sum();
}
}
And here are the snippets of the typical output:
# VM invoker: /Library/Java/JavaVirtualMachines/jdk1.8.0_20.jdk/Contents/Home/jre/bin/java
# VM options: <none>
# Warmup: 300 iterations, 200 ms each
# Measurement: 1 iterations, 1000 ms each
# Threads: 1 thread, will synchronize iterations
# Benchmark mode: Average time, time/op
# Benchmark: org.sample.Measure.multiply
# Run progress: 0,00% complete, ETA 00:01:01
# Fork: 1 of 1
# Warmup Iteration 1: 0,779 ns/op
# Warmup Iteration 2: 0,684 ns/op
# Warmup Iteration 3: 0,608 ns/op
# Warmup Iteration 4: 0,619 ns/op
# Warmup Iteration 5: 0,642 ns/op
# Warmup Iteration 6: 0,638 ns/op
# Warmup Iteration 7: 0,660 ns/op
# Warmup Iteration 8: 0,611 ns/op
# Warmup Iteration 9: 0,636 ns/op
# Warmup Iteration 10: 0,692 ns/op
# Warmup Iteration 11: 0,632 ns/op
# Warmup Iteration 12: 0,612 ns/op
# Warmup Iteration 13: 1,280 ns/op
# Warmup Iteration 14: 7,261 ns/op
# Warmup Iteration 15: 7,379 ns/op
# Warmup Iteration 16: 7,376 ns/op
# Warmup Iteration 17: 7,379 ns/op
# Warmup Iteration 18: 7,195 ns/op
# Warmup Iteration 19: 7,351 ns/op
# Warmup Iteration 20: 7,761 ns/op
....
....
....
# Warmup Iteration 100: 7,300 ns/op
# Warmup Iteration 101: 7,384 ns/op
# Warmup Iteration 102: 7,132 ns/op
# Warmup Iteration 103: 7,278 ns/op
# Warmup Iteration 104: 7,331 ns/op
# Warmup Iteration 105: 7,335 ns/op
# Warmup Iteration 106: 7,450 ns/op
# Warmup Iteration 107: 7,346 ns/op
# Warmup Iteration 108: 7,826 ns/op
# Warmup Iteration 109: 7,221 ns/op
# Warmup Iteration 110: 8,017 ns/op
# Warmup Iteration 111: 7,611 ns/op
# Warmup Iteration 112: 7,376 ns/op
# Warmup Iteration 113: 0,707 ns/op
# Warmup Iteration 114: 0,828 ns/op
# Warmup Iteration 115: 0,608 ns/op
# Warmup Iteration 116: 0,634 ns/op
# Warmup Iteration 117: 0,633 ns/op
# Warmup Iteration 118: 0,660 ns/op
# Warmup Iteration 119: 0,635 ns/op
# Warmup Iteration 120: 0,566 ns/op
The key moments happen at iterations 13 and 113: first the performance is degraded by a factor of ten, then it is restored. The corresponding times are 2.5 and 22.5 seconds into the test run. The timing of these events is very sensitive to the array size, BTW.
What can possibly explain this sort of behavior? The JIT compiler had probably done its work within the first iteration; there is no GC action to speak of (confirmed by VisualVM)... I am at a complete loss as to any kind of explanation.
My version of Java (OS X):
$ java -version
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)
org.sample.Measure::multiply, is getting recompiled around that time. – Astronomer